1. Field of the Invention
The present invention relates to motion detection systems, and, more particularly, to motion detection systems using passive infrared (PIR) motion sensors.
2. Description of the Related Art
It is known that all objects transmit a level of infrared light that varies with the temperature of the object. Taking advantage of this characteristic, passive infrared (PIR) motion sensors are used in security systems to detect motion of a relatively warm body that emanates a relatively high level of infrared light, such as a human intruder or motor vehicle. The sensors monitor the level of infrared light emanating from each of a plurality of detection zones. If the level of infrared light in any of the detection zones suddenly increases by a significant amount, as detected by the motion sensors, then the motion sensors transmit an alarm signal. The alarm signal indicates that the motion sensor has sensed the motion of a warm body.
A problem is that the pyroelectric sensing elements used in PIR motion sensors are sensitive to broad band visible light as well as to infrared light. Thus, it is possible for visible light to be interpreted by the PIR motion sensor as infrared light, thereby causing the sensor to issue a false alarm. Visible light produced by car headlights and handheld flashlights are typical false alarm sources.
It is known to add a multilayer silicon filter to the pyroelectric sensing element package in order to reduce the amount of visible light that reaches the pyroelectric sensing element. However, some small amount of visible light still passes through the filter. Additionally, some of the visible light illuminating the filter is converted and reradiated as infrared light. The polyethylene fresnel lens or window of the optical assembly of the motion sensor is commonly impregnated with pigments in order to provide additional filtering. Even with these measures, the PIR motion detector is subject to issuing false alarms due to visible light levels ranging from a few hundred lux to several thousand lux. Including more than one multilayer silicon filter or adding more pigment to the fresnel lens beyond an optimal amount results in a reduction of the sensitivity of the motion detector to the infrared light and impairs the overall performance of the motion detector.
Moreover, many countries have regulations that require that a motion detector be immune to visible light up to 6,500 lux, which is approximately the level of light produced by a car headlight aimed at the PIR sensor at a distance of ten feet. If a motion detector does not comply with such regulations, it will likely be barred from being sold within the country in which the regulations are in effect.
What is needed in the art is a motion detection system that is not susceptible to issuing false alarms due to the presence of visible light.